Liquid crystal displays (LCDs) have been commercially available for years, but until recently were generally restricted to relatively small sizes. Such displays were widely used originally in, for example, watches, calculators, radios, and other products requiring character and/or image indicators in relatively small display areas. Attempts to employ larger area LCDs initially provided such unacceptable levels of contrast, cost, and other characteristics as to render them commercially infeasible.
Recently, improvements in liquid crystal display technology have allowed larger active (AMLCD) and passive (PMLCD) matrix type displays to be commercially manufactured. AMLCDs, for example, have achieved widespread acceptance in portable computers, laptop computers, word processors, and avionic cockpit applications. Other products in which such displays are useful include flat screen and projection television systems. In each of the above LCD applications, it is desirable to have: high contrast ratios over a large viewing zone or envelope; little or no inversion; low amounts of ambient light reflectance; high resolution; and little or no color shifting over a wide range of viewing angles. However, when going to these larger displays where these characteristics are most needed, they have not always been capable of optimization. Indeed, one characteristic heretofore may have had to have been traded off against another in order to achieve commercial acceptability.
Liquid crystal materials are useful for such displays because light traveling therethrough is affected by the anisotropic or birefringent value (.DELTA.n) of the LC material, which in turn can be controlled by the application of a variable voltage across the LC. Backlit liquid crystal displays are desirable because the transmission of light emanating from the backlight assembly can be controlled with a substantially low amount of power, i.e. less than that typically required to illuminate other types of prior art displays such as CRTs. Furthermore, the thin profile of such LCDs gives them an added advantage over conventional CRTs.
The information displayed by, for example, AMLCDs is presented in the form of a matrix array of rows and columns of numerals or characters, which are generated by a number of segmented electrodes arranged in a matrix pattern. The segments are connected by individual leads to driving electronics, which act to apply a variable and controllable voltage to the appropriate combination of segments to thereby display the desired data and information by controlling the light transmitted through the LC material. Graphic information in, for example, avionic cockpit applications or television displays may be achieved by an active matrix of pixels which are connected by an X-Y sequential addressing scheme between two sets of perpendicular conductive lines (i.e. row and column address lines). Current advanced addressing schemes use conventional arrays of, for example, amorphous silicon (a-Si) TFTs, amorphous silicon thin film diodes, MIMs, etc., which act as switching elements to control the variable driving voltages at individual pixels (or colored subpixels). These schemes are applied to both twisted nematic (TN) and ECB (electrically controlled birefringence) liquid crystal displays as well as other conventional types. In a similar manner, STNs are generally multiplexed in order to selectively address each pixel.
Contrast ratio, ambient light reflection, gray level behavior, resolution, color shifting, and inversion, all of which have conventional definitions well-known in the art, are important attributes determining the quality of liquid crystal displays of all types.
The contrast ratio (i.e. contrast--1) in a NW display, for example, is the difference between "off state" transmission versus "on state" transmission in low ambient conditions, and is determined by dividing the "off state" light transmission (high intensity white light) by the "on state" or darkened intensity. For example, if an "off state" (i.e. below the threshold driving voltage) NW display exhibits an intensity of 200 fL and the same display in its fully driven "on state" emits 5 fL at the same viewing angle that the aforesaid 200 fL measurement was taken, both in low ambient conditions, then the display's contrast ratio at that particular viewing angle is 40 or 40:1. Accordingly, in normally white LCDs, the primary factor limiting the contrast ratio is the amount of light which leaks through the display in the darkened or "on state". In normally black LCDs, the primary factor limiting the contrast ratio achievable is the amount of light which leaks through the display in the darkened or "off state". The higher and more uniform the contrast ratio of a display over a large range of viewing angles, the better the LCD. Contrast ratio problems are compounded in bright environments, such as sunlight and other high intensity ambient conditions, where there is a considerable amount of reflected and scattered ambient light adjacent the display.
The lesser the amount of ambient light reflected from the display panel, the better the viewing characteristics of the display. Therefore, it is desirable to have an LCD reflect as little ambient light as possible. The amount of ambient light reflected by a display panel is typically measured via conventional specular and diffused reflection tests discussed later herein and illustrated by FIGS. 23 and 24.
In color LCDs, the aforesaid light leakage often causes severe color shifts for both saturated and gray scale colors. The shifting of such colors and/or images is particularly harmful when the display is to be viewed at increased or large viewing angles, such as in an avionic cockpit where the copilot's view of the pilot's displays is important. An example of color shifting is when a display pixel outputting the color navy blue at normal (0.degree. vertical, 0.degree. horizontal) clearly appears navy blue to the viewer at normal but appears either sky blue or purple when viewed at increased viewing angles (e.g. 0.degree. vertical, 45.degree. horizontal), these viewing angles being defined herein with reference to FIG. 22 and its corresponding description. It is highly desirable that an LCD substantially maintain color uniformity over a wide range of viewing angles so that one or more viewers see the same image no matter what viewing angle they are positioned at. Accordingly, the less color shifting in a display, the better its viewing characteristics.
Gray level performance of liquid crystal displays and the corresponding amount of inversion is also very important in determining the quality of an LCD. Conventional AMLCDs, for example, utilize anywhere from about 8 to 64 different driving voltages. These different driving voltages are typically referred to as gray level voltages. The intensity of light transmitted through the colored subpixel, pixel, or display depends upon its driving voltage. Accordingly, gray level voltages are used to generate different shades of different colors so as to create different colors when, for example, these shades are mixed with one another.
Preferably, the higher the driving voltage in, for example, a normally white twisted nematic display, the lower the intensity (fL) of light transmitted therethrough. Likewise then, the lower the driving voltage in such a normally white display, the higher the intensity of light emitted therefrom. The opposite is true in normally black twisted nematic displays.
Thus, by utilizing multiple gray level driving voltages, one can manipulate, for example, either a normally white (NW) or normally (NB) black display pixel (or colored subpixel) to emit a desired intensity of light. A gray level V.sub.on is any voltage greater than V.sub.th (threshold voltage) up to about 5.0 to 6.5 volts.
In conventional LCDs, inversion often adversely affects the aforesaid described gray level performance of the display, inversion being discussed and defined later herein. It is desirable in gray level performance of, for example, NW displays to have an intensity versus driving voltage curve wherein the intensity (i.e. fL) of light emitted from the pixel or subpixel continually and monotonically decreases as the driving voltage increases. In other words, it is desirable to have gray level performance in a NW pixel such that at all viewing angles the intensity of light emitted at 6.0 volts is less than that at 5.0 volts, which is in turn less than that at 4.0 volts, which is less than that at 3.0 volts, which is in turn less than that at 2.0 volts, etc. The opposite is true with respect to NB displays.
Inversion occurs in a NW display when the intensity (fL) at certain viewing angles at e.g. 3.0 volts is greater than that at 2.0 volts. This leads to different intensities and/or colors of light being viewed at various viewing angles of the display even when the same voltage is being applied. Accordingly, the elimination of inversion in LCDs is an always desired result. The problems of inversion are more thoroughly discussed in co-pending commonly owned Ser. No. 08/167,652, filed Dec. 15, 1993, the disclosure of which is incorporated herein by reference.
Normally black twisted nematic displays typically have better contrast ratio contour curves or characteristics than do their counterpart normally white displays. However, normally black (NB) displays are much harder to manufacture than NW displays due to their high dependence on the cell gap or thickness "d" of the liquid crystal material. Accordingly, a long felt need in the art, particularly in the art of manufacturing larger LCDs, has been the ability to construct a normally white display with high contrast ratios over a large range of both vertical and horizontal viewing angles, rather than having to resort to the more difficult to manufacture NB display to achieve these characteristics. While the subject invention is equally applicable to both NW and NB displays, one of its unique features is that it so successfully solves this long felt need by achieving at least those characteristics of NB displays in the simpler to construct NW displays provided according to this invention.
Heretofore, retardation films have been used in normally white displays in an attempt to enlarge their relatively small effective viewing areas and to reduce inversion. See, for example, U.S. Pat. No. 5,184,236 and aforesaid Ser. No. 08/167,652. These normally white TN displays with dual retardation films achieve fairly high contrast ratios over a relatively large range of viewing angles compared to other NW displays. Additionally, the reflectance of ambient light (e.g. sunlight) in both these displays is kept to a minimum by disposing the retardation films and LC material within opposing linear polarizers. Typically, an AR coating is provided exterior the front polarizer. Thus, both these displays achieve acceptable results with respect to both the size of their viewing zones and the amount of ambient light reflectance seen by the viewer. In the latter instance, improvements over the former with respect to the displays viewing characteristics are self-evident. While the solution of retardation films has proven efficacious, it would be desirable to eliminate the use of such films as well as to improve if possible upon the characteristics of the effective viewing zones, color shifting, contrast ratios, resolution, and inversion affecting gray scale levels over a wide range of viewing angles, particularly in the larger LCD sizes.
Prior to the subject invention, other attempts have been made to improve the viewing characteristics of liquid crystal displays by way of providing diffusers exterior the display's front polarizer. See, for example, U.S. Pat. Nos. 4,171,874; 4,704,004; and 5,046,827. The displays of these patents include light diffusers positioned in the view path of the display exterior the display's front polarizer. The sought after result is apparently an enlarged viewing zone. However, these displays including diffusers disposed exterior the front polarizer, or between the viewer and the front polarizer, typically experience a problem with respect to ambient light reflection. At times and in addition, color shifting, resolution, and inversion are less than optimal.
It is apparent from the above that there exists a long felt need in the art for a liquid crystal display (normally white, normally black, active, passive, TN, STN, etc.) for outputting high contrast ratio images over an increased range of viewing angles to a remotely positioned viewer, the display maintaining color uniformity over a wide range of viewing angles and having high resolution, relatively low ambient light reflectance, and little or no inversion.